1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a complementary metal-oxide-metal (CMOS) image sensor and a method of fabricating the same. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for enhancing light focusing characteristics.
2. Discussion of the Related Art
Generally, an image sensor is a semiconductor device, which converts an optic image to an electric signal. More specifically, a charge coupled device (CCD) is a device having a plurality of metal-oxide-metal (MOS) capacitors each formed within a proximate range from one another, and wherein a carrier electric charge is stored in and transmitted to each capacitor. A complementary MOS (CMOS) image sensor is a device forming a number of MOS transistors corresponding to the number of pixels by using the CMOS technology, which uses a control circuit and a signal processing circuit as peripheral circuits, and adopting a switching method, which uses the MOS transistors to sequentially detect each output.
However, the CCD is disadvantageous in that the driving method is complicated and consumes a large amount of power, and the fabrication process is complicated having too many mask process steps. Furthermore, a one-chip circuit cannot be easily formed because the signal processing circuit cannot be realized in the CCD chip. Recently, in order to overcome such disadvantages, a CMOS image sensor using sub-micron CMOS fabrication technology has been under development. The CMOS image sensor includes a photodiode and a MOS transistor formed within the unit pixel and uses a switching method to sequentially detect each signal, thereby representing an image. Herein, power consumption can be reduced by using the CMOS fabrication technology, and the number of masks is reduced to 20, as compared to the CCD fabrication process requiring 30 to 40 masks, thereby simplifying the fabrication process. Also, a plurality of signal processing circuits and one-chip circuits can be formed. For such reasons, the CMOS image sensor is being considered as the next generation image sensor.
FIG. 1 illustrates a circuit diagram of a unit pixel of a general CMOS image sensor, which includes a photodiode (PD) and four MOS transistors. The unit pixel includes a photodiode (PD) receiving light so as to form photocharge, a transfer transistor (Tx) transferring photocharge collected in the photodiode (PD) to a floating diffusion (FD) area, a reset transistor (Rx) setting an electrical potential of the floating diffusion (FD) area to a desired value and outputting electric charge, thereby resetting the floating diffusion (FD) area, a drive transistor (Dx) acting as a source follow buffer amplifier, and a select transistor (Sx) allowing addressing of the switching function. Also, a load transistor for reading an output signal is formed outside of the unit pixel.
FIG. 2 illustrates a cross-sectional view of a unit pixel of a related art CMOS image sensor and, more particularly, the main parts related to light-focusing. Referring to FIG. 2, the related art CMOS image sensor includes a semiconductor substrate 11, a field oxide layer 12 formed on the semiconductor substrate 11 to define an active area, and a photodiode (PD) 13 and a plurality of transistors 14 formed in the active area of the semiconductor substrate 11. Then, an interlayer dielectric 15 is formed on the entire structure including the photodiode (PD) 13 and the transistors 14. And, a plurality of metal circuits 16, 17, and 18 is formed on the interlayer dielectric 15 so as to form the unit pixel. The metal circuits 16, 17, and 18 are aligned so as to prevent light being irradiated to the photodiode (PD) from being blocked.
Interlayer dielectrics 19, 20, and 21 are formed between each layer of metal circuits 16, 17, and 18 for electric insulation. And, although not shown in the drawing, a passivation layer for protecting the device from humidity and scratches, R, G, and B color filter layers for representing a color image, and an overcoating layer are formed on the last interlayer dielectric 21. And, a micro-lens 22 having a dome structure and formed of a photoresist material is formed on the overcoating layer. The micro-lens 22 deposits a photoresist layer on the overcoating layer, which is patterned so that the photoresist remain on the photodiode (PD). Then, the photoresist is treated with a baking process so as to induce the photoresist to leak, thereby obtaining a desired curvature. The above-described micro-lens 22 plays an important role of focusing the irradiated light and transmitting the focused light to the photodiode (PD). However, as high density devices are being developed, the metal circuits should be formed in different layers, thereby increasing the thickness of the interlayer dielectrics. Thus, the micro-lens 22 becomes insufficient for appropriately focusing and transmitting light to the photodiode (PD).
FIGS. 3A to 3D illustrate problems shown in the related art CMOS image sensor. Referring to FIG. 3A, the related art CMOS image sensor the focused light passing through the micro-lens 22 is concentrated only on the center portion of the photodiode (PD), which causes a difference in amount of light between the edge portion and the center portion, thereby generating a dark current in the edge portion of the photodiode. Thus, when the irradiated light is converted into an electric signal, as shown in FIG. 3B, and outputted through a display device, the picture image is distorted at the edge portion and the clearness of the image is deteriorated.
Furthermore, referring to FIG. 3C, when the light is not irradiated in a direction parallel to the axis of the micro-lens and is irradiated to have an inclination, the light cannot be accurately focused on the photodiode and is focused more on another area, thereby reducing the intensity of the signal. Therefore, as shown in FIG. 3D, the outputted image may be focused to one side, and problems of distorted image and unclearness at the edge portion may occur. As the size of the unit pixel is reduced, the thickness of the upper structures, such as the interlayer dielectrics 15, 19, 20, and 21 and the metal circuits 16, 17, and 18 formed on the photodiode, are increased, as compared to the size of the photodiode. Such problems are aggravated as the number of metal circuits increases and the thickness of the structure is increased due to the high integration of the device, thereby causing problems in device integration. This problem may be resolved if the micro-lens 22 is formed on a lower portion, instead of the uppermost portion. However, if a later (or following) process is performed after forming the micro-lens 22 using the photoresist as material, the photoresist may leak due to the heat of the following process, and so such method cannot be actually applied.